This invention relates to Dense Wavelength Division Multiplexer (DWDM) devices, and more particularly to DWDM cascade tree structures.
A DWDM device can be used to increase the number of communication channels available in a fiber optical system. Today, researchers are studying a few competing technologies. One mature technology, relying on standard filter coatings, is characterized by high signal insertion losses and relatively high cost and is generally useful only for DWDM devices with channel spacing greater than 50 GHz.
A competing technology uses Mach-Zehnder interferometers (xe2x80x9cMZIsxe2x80x9d) and is characterized by low signal insertion loss, low polarization dependent low, relatively low cost, high uniformity and low signal crosstalk and is a more attractive choice for DWDM devices with lower channel spacing. However, standard MZI technology suffers from low isolation between adjacent channels and relatively high passband insertion loss, caused in part by instability of the light source. These latter two problems make it difficult for a DWDM device relying only on standard MZI technology to comply with DWDM standards (optical isolation, flat-top response) set down by BellCore. If these two problems can be either solved or reduced in severity, DWDM devices relying on MZI technology could become widely used in voice, data and image communications.
What is needed is a DWDM system having low signal insertion loss, low polarization dependent loss, high uniformity, relatively low signal crosstalk, acceptable channel isolation and acceptably low passband insertion loss. Preferably, the system should have acceptably low cost and should be flexible enough to meet various commercial communication requirements. Preferably, the system should meet the BellCore standards for optical isolation, for xe2x80x9cflat-topxe2x80x9d wavelength response and for crosstalk suppression.
These needs are met by the invention, which uses an improved DWDM cascade tree structure with distributed filtering and MZI technology to provide acceptable channel isolation for relatively low channel spacing and to comply with the BellCore standards for optical isolation, for flat-top passband response and for crosstalk suppression within the channels, over a system of 2N output channels for 50 GHz channel spacing with N=5, 6, 7, . . . The basic structure is a bifurcated or cascade tree system with N+1 stages, numbered n=0, 1, 2, . . . , N, with stage 0 being a light input channel, with stage number n having 2n fiber optical channels in parallel, with each channel in stage n having an MZI, defined by two 3 dB couplers and two parallel fiber optic arms of unequal length, at the beginning of the channel, and with each channel except an output channel or port feeding an MZI that is part of stage n+1 (n=1, 2, . . . , Nxe2x88x921). A xe2x80x9cstagexe2x80x9d, as used herein, refers to a group of one or more parallel fiber optic channels, with each channel having an MZI positioned at the beginning of the channel for wavelength discrimination. A typical cascade tree structure of fiber optic channels is disclosed and discussed in U.S. Pat. Nos. 5,809,190 and 5,987,201, issued to P. Z. Chen (FIG. 1 and discussion), incorporated by reference herein.
In an embodiment of the invention, each channel in each stage, except an input channel, includes an MZI, or an MZI combined with a phase shift mechanism, for low signal insertion loss and wavelength discrimination within that channel.
The channels in three independently selected stages, preferably the first three stages, include a combined MZI and phase shift mechanism (xe2x80x9cPSMZIxe2x80x9d), positioned at the end of each channel. The phase shift mechanism may be a Gires-Tournois optical resonator, or any similar optical resonator, that transmits all lightwave energy it receives, but with an additional, controllable phase shift that depends upon wavelength, refractive index of a separator material, and separation distance of two reflecting surfaces of the resonator.
By introducing an additional, controllable phase shift into light transported within the cascade tree structure, light is processed to comply with the BellCore standards for optical isolation, for flat-top passband response and for crosstalk suppression at the output channels.